Modular Synth Panels

Wow, good work Mark!

Thanks for all the info -- very handy.

Cheers
Scott Deyo
The Bridechamber
contact@...
http://www.bridechamber.com
Jealous Edison Record Kompany
http://www.jealousedison.com


On May 21, 2008, at 2:35 PM, Mark wrote:

>
>  I recently completed construction and initial testing of the CGS tube 
> VCA
>  using a 1SH24B tube. I built it in MOTM-format using a panel and
>  bracket from Bridechamber.
>
>  The version I built is different than the original circuit. In order
>  to avoid damaging the PCB from excessive unsoldering, I did not first 
> build
>  a stock unmodified version for comparison purposes.
>
>  This is taken from posts I made to the cgs_synth and tubesynthdiy
>  yagoogroups where I went into more detail as far as my reasons for
>  doing things. Since no one posted any corrections, I'd thought I'd
>  go ahead and post it here.
>
>  From what others had said, there were three issues: the output was
>  noisy, there was a "high-pass effect" or "loss of bass", and there
>  was a "thump" during the attack of the EG when used as a VCA. This
>  is a list of changes designed to address those issues:
>
>  I eliminated the two signal diodes, the two LED's, and their 1.5K
>  resistors. I connected a 1.2K 1/2W resistor between Pin 1 of the
>  cathode-heater and ground.
>
>  I added a a 47uF axial electrolytic cathode bypass capacitor between
>  pin 2 of the tube (connected to the negative rail) and ground. Do
>  not connect it to the ground side of the cathode or the in-rush
>  current could burn out the heater.
>
>  I replaced the TL072 with an OP275. I eliminated the stock 100nF
>  bypass caps, and underneath the PCB, added two .1uF ceramic caps with
>  heat shrink tubing going from ground, with the capacitors as close as
>  possible to the power pins on the socket. I eliminated the 47pF cap
>  on the output, and replaced it with a resistor lead. I did not add
>  the gain resistor. While 330 Ohm is the minimum specified resistor
>  for output protection, I replaced it with 1K (outside the feedback
>  path) to have the same output impedance as MOTM.
>
>  I increased the two 220nF coupling caps to 470nF.
>
>  I decreased the CV input resistor to 50K so a 0-5V EG it has about 
> the same
>  range as the Gain knob.
>
>  I wired up the SPDT bias switch -- using properly shielded cable -- 
> to two
>  settings:
>
>  For a "clean" setting, I added a 20K multi-turn trimmer to the
>  board. There is an area with some writing without any traces to the
>  right of the -Ve hole for the stock 330K resistor, where I drilled
>  two extra holes. The trimmer is wired as a rheostat between -Ve and
>  the switch.
>
>  For a "distortion" setting, I connected a 475K resistor between
>  ground and the switch.
>
>  The suspression grid was connected to the negative rail.
>
>  I used Bournes conductive plastic 50K linear pots for all four knobs.
>
>  While it likely varies from 1SH24B to 1SH24B, the tube I have
>  installed is more than a bit microphonic. Clicking the bias switch
>  is enough to produce ringing. So I stuffed some anti-static foam in
>  between the tube and the board to minimize vibration.
>
>  I think that's it.
>
>  These modifications seem to have worked very well. I don't notice
>  any noise on the output.
>
>  On the clean setting, with a roughly 10Vpp input potted all the way
>  up, the distortion is barely noticeable. Three things are happening
>  as the value of the variable resistor is adjusted lower: the grid
>  becomes more negative, the level of the AC signal decreases, and the
>  frequency of the high-pass filter increases. As such, there is some
>  loss of bass, which rolls off gently at the lowest audible
>  frequencies. With or without a signal, the grid bias is equal to
>  -15V, due to the low impedance. I would suggest using a 100K
>  multiturn trimmer instead -- that way you could adjust a "clean"
>  setting with the input potted down a bit for slightly better bass
>  response (although the high-pass effect of the original circuit was
>  due to a lack of cathode bypassing, as the corner frequency of the
>  stock values of 220nF and 330K in parallel with the grid resistance
>  was below the audible range). You also might want to try even larger
>  values for the coupling caps. I used AVX BQ yellow box caps, which
>  go up to 1uF, but you should double-check the lead spacing.
> http://avx.com/docs/catalogs/bf-bq.pdf
>
>  The distortion setting produces obvious distortion, and can produce
>  oscillations and other noises at various input and feedback settings.
>  The low frequency response is well below the audio range (so the 
> module will
>  just about work as a depth control for a fast LFO). The AC voltage
>  on the grid is about 3.3 RMS. With a signal the grid measures around
>  -17.6 VDC (it varies with its frequency), and without a signal the
>  bias is around -14.1V.
>
>  While the LPF in the CV circuit is the same as the stock circuit, the
>  cathode bypass cap improves the transient response of the tube
>  itself. It seems likely that rapid changes in voltage on the control
>  grid (eg. an envelope with a steep attack) would cause a sudden
>  demand for plate current enough to deplete the cathode, which then
>  bounces back causing an unwanted noise.
>
>  There is a bit of a pop or click with a fast envelope, although it is
>  not any worse than most solid-state VCA's. Changing the signal grid
>  bias, and switching between the two settings, also effects its CV
>  response. As such, it is possible to all but eliminate any bleed,
>  such that there is only a boost at the initial transient for punchy
>  basses and leads. I found this while experimenting with different
>  values. It would increase the distortion from the "clean" setting,
>  but it would still be a very nice feature. I might add that if I can
>  find a suitable SP3T switch.
>
>  Regardless of the signal level on the grid, or the bias switch
>  setting, the output of the tube, and therefore the module, is a bit
>  below 1V RMS, as it seems that is the saturation limit of the tube.
>  So while the signal on the grid is much higher for the distortion
>  setting, the difference results in just that -- distortion. In fact,
>  the "clean" setting measures just a bit more, although the
>  "distortion" setting sounds louder to to the additional harmonics.
>  Now 1V RMS is only about 2.8Vpp. Although it is enough on the
>  board, I might change the gain and feedback resistors to 2K
>  (resulting in about 5.6Vpp) if it seems too low to work well with my
>  other modules.
>
>  For those building this in other formats that might have he extra
>  space, putting a bias rheostat on the panel would be a nice feature.
>  Using a reversing attenuator for the second input/feedback control is
>  another option, as negative feedback is also useful.
>
>  So with these modifications there is very little noise, sufficient
>  bandwidth, and it works fine both as a "regular" VCA and a
>  distortion/waveshaper module.
>
>  I am open to questions and comments.
>
>  
>